1. Field of the Invention
This invention relates to a fire alarm system, and more particularly to a fire alarm system wherein both power supply from a central signal station to fire detectors and transmission of a fire detection signal from the fire detectors to the central signal station are carried out by way of an optical transmission line using an optical fiber cable.
2. Description of the Prior Art
Recently, an optical fiber cable has been put to use in the field of a fire-guard system, with the advance of mass-production technique and application technique of the optical fiber cables.
The optical fiber cable has such an advantage that it enables signal transmission free from an electrical disturbing influences which have been problems to be solved in this field. By employing the optical fiber cable, a misoperation due to an electrical disturbance such as a strong electric field, a thunder surge, etc. can be eliminated. The optical fiber cable further has an advantage in heat resistance and corrosion resistance, as compared with conventional signal lines.
However, in the conventional fire alarm system, the optical fiber cable has been used only for signal transmission of a fire detection signal from a fire detector to a central signal station.
In this connection, it is to be noted that, in an ionization type smoke detector, power must be supplied to the detector from the central signal station, and, in general, a power supply line is used as a signal line, too. However, in the case where the optical fiber cable is used as the signal line, each of the detectors must be provided with a power source therein or a special line for power supply must be additionally provided according to a conventional technique. Thus, there have heretofore been caused such problems that the formation of the system is made complicated and the cost for installation of the system is increased by the employment of the optical fiber cables.
In the field of a measuring system, there has been proposed an arrangement in which a central station transmits an energy after converting it into light to a terminal equipment through an optical fiber cable and the terminal equipment converts the light into electric power to operate circuits contained therein for carrying out measurement and transmits the obtained data to the central station after converting it into light. In the measuring system, generally, a single terminal equipment is connected to the central station. Therefore, it is easy to transmit sufficient power light from the central station to the terminal.
However, in case of fire alarm system, a considerable number of detectors are connected to a single central station and the detectors are disposed remote from each other. By this reason, it is difficult to transmit sufficient light to the detectors simultaneously to operate them. Thus, the technique of the measuring system cannot be applied as it is to the field of fire alarm system.